Genetic engineering applied to cultivated plants is causing a new era in agriculture. The first generation of transgenic herbaceous plants with genes of agronomic interest are already on the market and new and better perspectives are being opened for the near future. The potential of genetic improvement by means of transformation is even more interesting in the case of woody species, as most commercial varieties propagate vegetatively and are hybrids of unknown origin which basically cannot be improved by classical genetic means due to their highly heterozygotous nature. Even when traditional improvement is possible, another important obstacle is the long time period between generations. Genetic engineering permits the insertion of specific genes in the unknown genetic background of commercial varieties, adding desirable characteristics to these plants without altering other characteristics of argonomic interest.
The improvement of these woody species by means of genetic transformation will be of limited applicability unless it becomes possible to establish procedures for the genetic transformation and regeneration of adult vegetable material. Until now, procedures for the transformation of woody plants have been restricted to young tissue derived from seeds, such as, for example, zygotic embryos, cotyledons or hypocotyls (Reference 1-15). However, the sexual process completely modifies the genome of plants proceeding from seeds, altering their argonomic characteristics. In other cases in which embryogenetic cells of somatic origin, somatic embryos or vegatatively propagated tissue were used as vegetable starting material, the explants were young (Reference 3, 7, 16-33) or were rejuvanted after successive micropropagations in vitro (Reference 34-39). If these procedures and this type of vegetable starting material were used to introduce genes of agronomic interest into woody species, the regenerated transgenic plants would have juvenile qualities, and would therefore require a period of several years before it would be possible to evaluate their agronomic characteristics. Moreover, if tissue derived from seeds were used, the transformed plants would not have the same agronomic genetic characteristics as the original plants. The careful evaluation of the agronomic characteristics of adult woody species is a requirement to contemplate the commercial exploitation of new varieties in large cultivated areas.
Therefore, it is necessary to avail of a method for the genetic transformation of woody species to overcome the above mentioned problems. This aim may be obtained by means of the procedure of this invention, which consists of the direct genetic transformation of cells proceeding from adult vegetable material and the regeneration of complete adult woody plants, which permits the transformation of commercial varieties with genes of agronomic interest, and to drastically reduce the period of time necessary to evaluate the agronomic characteristics of the new varieties obtained by means of the transformation technology, thus reducing the costs and also the time period in which the new varieties arrive to the farmers and consumers.